(1.) Field of the Invention
This invention relates to a biaxially oriented film mainly comprised of a polyester. More specifically, it relates to a biaxially oriented polyester film used as a substrate for magnetic recording media.
(2) Description of the Prior Art
A biaxially oriented polyester film is very useful as a substrate for magnetic recording media. Most magnetic recording media possess a laminate structure such that a magnetic recording layer is formed on a surface of a substrate film by means of coating, plating, or sputtering or other vapor deposition, and particularly, all of the currently widely accepted audio tapes and video tapes possess such a laminate structure. Regardless of the means whereby the magnetic recording layer is formed on the substrate film, it is common to all magnetic recording media that the substrate film plays an important role for the improvement in the characteristics of the magnetic recording media, particularly the electromagnetic transducing characteristics and running characteristics thereof.
Some proposals have heretofore been made for enhancing the abrasion resistance and runnability of a magnetic recording medium by incorporating in the substrate inert additives or both inert additives and internal particles defined as particles which have been formed by the reaction of an additive with the monomer or oligomer of the polyester (for example, refer to U.S. Pat. Nos. 3,821,156, 4,067,855 and 4,138,386 and British Patent No. 1,359,892). These proposed techniques provide substrate films having an increased number of surface protrusions possessing an enhanced height, which result in magnetic recording media of enhanced abrasion resistance and runnability. However, the substrate films are poor in surface smoothness and consequently in electromagnetic transducing characteristics.
It also has been proposed as described in U.S. Pat. No. 3,958,064 to prepare a two-layer coextruded film comprised of a relatively rough back layer containing polymeric particles and a relatively smooth face layer containing no particulate matter and to form a magnetic recording layer on the smooth face layer. It has been found, however, that the roughness of the back layer is transferred onto the smooth magnetic recording layer during the period when the magnetic recording tape is held in roll form, with the result that the desired degree of electromagnetic transducing characteristics cannot be attained.
Thus, no magnetic recording media have heretofore been put to practical use, which have satisfactory runnability and electromagnetic transducing characteristics both being at a high level of the extent desired in recent years.
By the term "runnability" as used herein is meant both frictional resistance and abrasion resistance. The term "protrusions" used herein means fine projections appearing on a surface of a film, which are dominated by the size and amount of particles incorporated in the film. The term "electromagnetic transducing characteristics" means characteristics expressing the level of a reproduced signal/recorded signal ratio and similar parameters and their variations, which characteristics include video signal-to-noise (S/N) ratio, chroma signal-to-noise (S/N) ratio, dropout and envelope, and their variations.